As digital communications networks become more advanced, various chip makers and equipment maker's continue to improve and advance the devices, methods and systems used to facilitate higher and higher data transfer rates using smaller and less costly equipment and devices.
For example, Cisco Systems, Inc. has developed an improvement to the media independent interface (“MII”) specification, which is known in the art for allowing a media access control (“MAC”) layer to control and interact with the physical interface (“PHY”) layer regardless of the type of physical media being controlled. The improvement is known in the art and defined by the Serial-MII (“SMII”) specification.
SMII specifies that instead of using a conventional seven-wire arrangement for transferring Ethernet data between MAC and a corresponding PHY layer components, time division multiplexing (“TDM”) techniques can be used to transport the same amount of data over two wires serially. This is accomplished by using a global clock signal to provide timing to a plurality of MACs and corresponding PHYs. In addition, a global sync signal is sent to the MACs and PHYs. Thus, each group (typically comprising eight MAC-PHY sets) of components need only have 4 pins/wires instead of the nine per MAC-PHY set used in a conventional Ethernet system.
While fewer pins and wires are required to connect the MACs to the PHYs under the SMII specification, the MACs and PHYs are inherently required to be located proximate one another, approximately within 1.5 ns. In other words, using SMII, MAC and corresponding PHY components should realistically be located on the same printed circuit board (“PCB”). This is due to trace delay caused by propagation characteristics of the connecting medium, such as copper.
To allow greater distances separation distances between the MAC and the PHY layers, a dedicated set of clock and sync signals may be used for the transmit direction and a separate set of dedicated signals may be used for the receive direction. This allows separation distances of the MAC layer devices from the PHY layer devices greater than the trace delay inherent in the SMII specification, while providing full duplex capability as well. This specification using separate signal sets for the transmit and receive directions respectively is known in the art as source synchronous serial media independent interface (“SSMII”).
Application of an SSMII system may be useful in computer network systems, telephony systems or any other type of system that transmits and receives digital data using the Ethernet format. As shown in FIG. 1, a typical Ethernet system 2 may comprise a plurality of computers 4A-n, which are connected together through network 6, typically an optical fiber network. Each computer 4A-n typically interfaces through nodes (interface devices) 8A and 8B. It will be appreciated that network 2 may comprise many more computers 4 and interfaces 8 than shown in the figure. Each of the interface devices 8 typically comprises a PHY 10, a MAC 12 and a switch 14. PHY 10 is typically selected to provide an interface between the MAC, an electrical device, and the computer 4, which may connect electrically, optically or wirelessly, to the network 6. Switch 14 typically performs routing and signal flow functionality, i.e., which computer to route incoming signals to, and manage which connected computer (or other device) provides an outgoing signal at a given time. For example, if computer 4A is at a head end and computers 4B-n are subscribers, computer 4B may not be allowed to communicate directly with computer 4C, the communication there-between being routed through network 6 back to the head end computer 4A. Thus, computer 4A can be used to provide security and monitoring, and other management functions. These management functions are often performed by a management computer 16 at headend 15 with computer 4A functioning as a data server. Whatever the management arrangement, each switch at each computer 4A-n is managed independently of the others. In addition to signal flow control it is often desirable to be able to determine whether a particular customer or subscriber has a computer (or other network device) connected to the network and to be able to determine whether that subscriber device is transmitting or receiving a signal. When an apparent malfunction has occurred and a customer needs assistance, it is often necessary for service provider personnel to physically drive to the node location that houses interface device 8B to perform basic diagnostic routines, such as visually checking to see whether one of computers 4B-n are plugged into the network and/or are transmitting/receiving when they are supposed to be. In addition, each switch, MAC and PHY device, typically comprising integrated circuits mounted on a PCB, has a cost associated with it.
Thus, there is a need for a method and system for implementing an Ethernet network using SSMII technology that reduces the complexity of managing the signal flow through the switches, that reduces the need for personnel having to physically go to a site to perform rudimentary diagnostic functions, and that maintains low cost of the system by using off-the-shelf parts.